Mixture suited for crosslinking polymers and process for crosslinking polymers with extension of scorch time

ABSTRACT

The mixture consists of at least one organic peroxide, at least one hydroquinone derivative of the general formulae I and/or II and/or III ##STR1## wherein R and R 1 , which may be identical or different, stand for H or t-alkyl, and X stands, for example, for C 1-18  -alkylene, and a crosslinkage promoter, and permits the crosslinkage of polymers and at the same time extends the scorch time, which must be understood as the processing time.

This is a continuation of copending application Ser. No. 07/361,165,filed on Jun. 5, 1989, now abandoned.

DESCRIPTION

The invention relates to a mixture suited for crosslinking polymers andto a process for crosslinking polymers and at the same time extendingthe scorch time.

Organic peroxides are employed for crosslinking polyolefins andelastomers if the final products have to meet high mechanical andphysical requirements, such as improved thermal aging and lower residual(permanent) deformation under pressure, compared with sulfur-crosslinkedelastomers.

Crosslinking period, and the time from the addition of the organicperoxide to the polymer up to incipient cross-linkage (scorch time), aredependent on the thermal decomposition rate (half-life period) of theperoxides used as crosslinking agents. It is frequently desirable toextend the scorch time--(which must be regarded as processing period)without extending the crosslinking time--as is possible in sulfur-curingby the use of inhibitor and accelerator system in rubber--and withoutdeterioration of crosslinkage (resultant from degree of crosslinkage anddensity of crosslinkage).

It is possible in peroxidic crosslinkage to extend the scorch time bythe use of higher responsive crosslinkage initiators, i.e. initiatorswith longer half-life period; however, this is accompanied by a longercrosslinking period, and thus lower productivity.

Also the lowering of the process temperature allows the desiredextension of the scorch time, but again results in lower productivity.

The extension of the scorch time can also be realized by the use ofcertain auxiliary substances in the peroxidic crosslinkage of polymers:

German laid-open applications 2 553 145 and 2 553 094 describe the useof the hydroperoxides cumene hydroperoxide and t-butyl hydroperoxide asscorch time extenders in the peroxidic crosslinkage of ethylenehomopolymers and copolymers. The desired extension of the scorch time,however, is accompanied by an undesirable deterioration of thecrosslinkage.

British Patent No. 1,386,616 discloses the use of N-nitroso-diarylamines(e.g. N-nitroso-di-phenylamine) orN,N'-dinitroso-N,N'-diaryl-p-phenylene diamines (e.g.N,N'-dinitroso-N,N'-diphenyl-p-phenylene diamine) as retarding agents(=scorch inhibitors) in the peroxidic crosslinkage of rubber incombination with methacrylic or acrylic crosslinkage promoters (e.g.trimethylol propane-trimethacrylate). These nitroso compounds like--allN-nitroso compounds--are suspected to have a carcinogenic effect inhumans.

Y. W. Chow and G. Y. Knight, Rubbercon 77, Int. Rubber Conf. 1977, 2,contribution No. 24, describe the cross-linkage of rubber by means ofdicumyl peroxide in the presence of m-phenylene bis-maleinimide ascrosslinkage promoter and an radical scavenger, such asN-nitrosodiphenylamine, as scorch retarded. In addition to the desiredextension of the scorch time, however, the crosslinkage time isundesirably delayed.

U.S. Pat. No. 3,335,124 describes the peroxidic crosslinkage ofpolyethylene with the use of the following classes of substances asretarders to extend the scorch time: (a) antioxidants (inter alia also2,5-di-t-butyl hydroquinone and 2,5-di-t-amyl hydroquinone) or (b)compounds generally known as accelerators for the sulfur cross-linkageof rubber (e.g. 2-mercaptobenzothiazole, tetramethyl thiuram disulfide)or (c) compounds having both antioxidant function and acceleratorfunction in the sulfur curing of rubber (e.g. amine/aldehyde adductssuch as hexamethylene tetramine or butyraldehyde-monobutylaminecondensation product). Crosslinkage promoters are not used.

The Japanese Kokai Tokkyo Koho publication 59 215 347 (84 215 347)describes a process for crosslinking acrylic rubber by means of peroxide(dicumyl peroxide) as cross-linker in the presence of a crosslinkagepromoter (e.g. triallyl isocyanurate) and/or a thiourea derivative (e.g.trimethyl thiourea) and phenothiazine as scorch time extender.

According to British Patent No. 1,535,038 ethylene homopolymers orcopolymers are crosslinked by means of peroxide (e.g. dicumyl peroxide)as crosslinking agent in the presence of a crosslinkage promoter havingat least 3 allyl groups (e.g. triallyl cyanurate, triallyl phosphate,tetraallyl oxyethane) and an organic hydroperoxide (e.g. t-butylhydroperoxide, cumene hydroperoxide) as scorch time extender.

The object of the invention is the extension of the scorch time in thecrosslinkage of polymers with organic peroxides, while avoiding anextension of the crosslinking time and a derioration of thecrosslinkage; the scorch time extender is not to be volatile (likehydroperoxide), not toxic (like N-nitroso compounds) and is not tocontain sulfur (like phenothiazine) in order to avoid unpleasant odorsgiven off by the crosslinked final product.

This object is realized by the use of a combination of organic peroxide,a hydroquinone derivative, and a cross-linkage promoter.

The subject matter of the invention therefore is a mixture containing

(a) at least one organic peroxide suitable for crosslinking polymers,

(b) at least one hydroquinone derivative of the general formula I and/orII and/or III ##STR2## wherein R and R¹, which may be identical ordifferent, stand for H, t-alkyl, t-cycloalkyl, or aryl-t-alkyl andX=C₁₋₁₈ -alkylene optionally substituted by C₁₋₁₀ -alkyl and/or C₁₋₁₀-alkoxy and/or phenyl, C₂₋₁₈ -alkenylene or C₂₋₁₈ -alkpolyenylene bothof which may be substituted by C₁₋₁₀ -alkyl, or --(CR² R³)_(n)--Ar--(CR² R³)_(m) -- wherein R² and R³ are identical or different andstand for hydrogen or C₁₋₁₀ -alkyl, Ar is phenyl or naphthyl, n and mstand for 0 to 18, and n+m≦18, and X preferably stands for --CH₂CH═CHCH₂ --, --CH₂ --, CH₃ --CH<, (CH₃)₂ C<, or --C(CH₃)₂ --C₆ H₄--C(CH₃)₂ --, where the hydroquinones of the formula III are optionallysubstituted by C₁₋₁₀ -alkyl at the benzene ring of the naphthalenestructure which is not substituted by the hydroxy groups and R, and

(c) at least one crosslinkage promoter,

and a process for crosslinking polymers with extension of the scorchtime, especially of polyolefins and elastomers, by the addition to thepolymers to be crosslinked of

(a) at least one organic peroxide suited for crosslinkage of polymers,

(b) at least one hydroquinone derivative of the general formulae Iand/or II and/or III ##STR3## wherein R and R¹, which may be identicalor different, stand for H, t-alkyl, t-cycloalkyl or aryl-t-alkyl, andX=C₁₋₁₈ -alkylene optionally substituted by C₁₋₁₀ -alkyl and/or C₁₋₁₀-alkoxy and/or phenyl, C₂₋₁₈ -alkenylene or C₂₋₁₈ -alkpolyenylene bothof which may be substituted by C₁₋₁₀ -alkyl, or --(CR² R³)_(n)--Ar--(CR² R³)_(m) --, wherein R² and R³ are identical or different andrepresent hydrogen or C₁₋₁₀ -alkyl, Ar stands for phenyl or naphthyl, nand m are 0 to 18, and n+m≦18, and wherein X preferably is --CH₂CH═CHCH₂, --CH₂ --, CH₃ --CH<, (CH₃)₂ C< or --C(CH₃)₂ --C₆ H₄ --C(CH₃)₂-- where the hydroquinones of formula III are optionally substituted byC₁₋₁₀ -alkyl at the benzene ring of the naphthalene structure notsubstituted by the hydroxy groups and R, and

(c) at least one crosslinkage promoter

either separately or simultaneously, or successively, or in the form ofa mixture. In case of separate addition of the components the peroxideis normally added last.

The combination used according to the invention crosslinks a polymer inthe same time and with equally good cross-linking effect as the peroxidealone, while it provides the desired longer scorch time.

The mixtures according to the invention possess unexpectedly goodstorage stability despite the incompatibility of peroxide andhydroquinone derivatives, and of peroxide and crosslinkage promoters,during storage.

The polymers employed preferably are polyolefins and elastomers, namelyhomopolymers and copolymers amenable to crosslinkage by peroxides anddescribed in "Rubber World", October 1983, pages 26 to 32, in thearticle "Crosslinkage of Elastomers" and in "Rubber and Plastics News",September 1980, pages 46 to 50, in the article "Organic Peroxides forCrosslinkage of Rubber". These include, inter alia, the followingpolymers: low density polyethylene (LDPE), medium density polyethylene(MDPE), high density polyethylene (HDPE), linear low densitypolyethylene (LLDPE), linear medium density polyethylene (LMDPE),ethylene/propylene/diene terpolymer (EPDM), ethylene/propylenecopolymer, 1,4-polybutadiene, butadiene/styrene rubber,butadiene/styrene/acrylonitrile rubber, chlorinated polyethylene,chlorosulfonated polyethylene (=Hypalon), silicone rubber, nitrilerubber, butyl rubber, fluorinated rubber and ethylene/vinyl acetatecopolymer (EVA). Also mixtures of these polymers can be used.

Organic peroxides in the sense of the present invention are peroxidesforming corsslinkage-active free radicals upon thermal decomposition.Preferred peroxides are dialkyl peroxides and ketal peroxides, asdescribed in "Encyclopedia of Chemical Technology", 3rd Edition, Vol.17, pages 27 to 90.

The group of dialkyl peroxides used as preferred initiators includes,for example, dicumyl peroxide, di-t-butyl peroxide, t-butyl cumylperoxide, 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane,2,5-dimethyl-2,5-bis(t-butylperoxy)-hexine-(3),alpha,alpha'-bis(t-butylperoxy)-diisopropylbenzene, di-t-amyl-peroxide,1,3,5-tris(2-t-butylperoxy-isopropyl) benzene, 1-phenyl-1-t-butylperoxyphthalide.

The group of ketal peroxides, being likewise preferred initiators,includes, for example,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,1,1-bis(t-butylperoxy) cyclohexane, 2,2'-bis(t-butylperoxy) butane,ethyl-3,3-bis(t-butylperoxy) butyrate, n-butyl-4,4-bis(t-butylperoxy)valerate, and others.

From the group of peresters t-butyl-peroxybenzoate andt-butylperoxy-o-methyl benzoate are mentioned.

As representatives of mixed dialkylperoxide perketals3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxanonane and3,6,6,9,9-pentamethyl-3-carbethoxy-1,2,4,5-tetraoxanonane are mentioned.

Also other peroxides can be used as free radical source, and mixtures ofdifferent peroxides may be employed.

The peroxides are used in quantities from 0.1% to 15%, based on thepolymer, especially from 1% to 10%, preferably 1% to 8%.

In the hydroquinone derivatives of the general formulae I, II and IIIused according to the invention R and R¹ preferably stand for t-butyl,t-amyl, t-hexyl (=1,1-diethylethyl or 1,1-dimethylbutyl), t-octyl(=1,1,3,3-tetramethylbutyl), 1-methylcyclohexyl-(1); α-cumyl orp-isopropylcumyl, and X stands preferably for butanediyl-(1,4),octanediyl-(1,8), octatriene-(2,4,6)-diyl-(1,8), PhCH<, Ph(CH₃)C<, (Ph)₂C<, --CH₂ CH═CH--CH₂ --, --CH₂ --, CH₃ CH<, (CH₃)₂ C<, meta orpara--C(CH₃)₂ --C₆ H₄ --C(CH₃)₂ --.

Examples of hydroquinone derivatives of the formulae I, II and III aremono-t-butyl hydroquinone, 2,5-di-t-butyl hydroquinone,mono-t-amyl-hydroquinone, 2,5-di-t-amyl hydroquinone, mono-(or2,5-di)-t-hexyl hydroquinone, mono-(or 2,5-di)-t-octyl hydroquinone,mono-(or 2,5-di)-1-methylcyclohexyl-(1)-hydroquinone, mono-(or2,5-di)-cumyl hydroquinone; 1,4-bis(2-t-butylhydroquinonyl-(5))-butene-(2), alpha,alpha'-bis(2-t-butylhydroquinonyl-(5)) diisopropylbenzene; 2-t-butyl-5-t-amyl hydroquinone,2-t-butyl-5-t-octyl-hydroquinone; 2-t-butyl naphthohydroquinone,2-t-amyl naphthohydroquinone.

Mono-t-butyl hydroquinone (I, R=t-butyl, R¹ =H) or 2,5-di-t-amylhydroquinone (I, R=R¹ =t-amyl) are especially preferred.

The scorch time extenders are used in quantities from 0.1% to 10%, basedon polymer, especially from 0.2% to 2%, preferably 0.1% to 0.5%.

The crosslinkage promoters used according to the invention preferablyare compounds containing at least two polymerizable C--C double bonds.They are bi-functional and polyfunctional monomers. They include all di-and polyacryl, methacryl, vinyl, allyl, and methallyl monomers such astriallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallyltrimellitate, triallyl citrate, diallyl terephthalate, diallyl adipate,diallyl maleinate, diallyl fumarate, trimethylpropane trimethacrylate,ethylene glycol dimethacrylate, trimethylolpropane triacrylate, andm-phenylene bismaleinimide.

Also mixtures of various crosslinkage promoters can be used.

The crosslinkage promoters are used in quantities of from 0.1% to 15%,based on polymer, especially from 0.1% to 5%, preferably 0.2 to 2%.

The mixing ratio of peroxide to scorch time extender to crosslinkagepromoter can vary within wide limits, namely peroxide from 5 to 90%,preferably 20 to 90%, especially 20 to 50%; scorch time extender from0.1 to 50%, preferably 1 to 30%, especially 2 to 20%, and cross-linkagepromoter from 0.1 to 90%, preferably 1 to 30%, especially 2 to 20%,based on the total weight of (a) +(b)+(c).

Examples for optimum mixing ratios are

75% 1,1-bis(t-butylperoxy-3,3,5-trimethylcyclohexanone

8% 2,5-di-t-amyl hydroquinone

17% triallyl trimellitate

73% 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane

7% 2,5-di-t-amyl hydroquinone

20% triallyl trimellitate

73% dicumyl peroxide

6.5% di-t-butyl hydroquinone

19.5% triallyl cyanurate

The mixture according to the invention may be liquid and may containinert solvent as fourth component. It may be a powder and may contain asfourth component inert powdered fillers (e.g. silica, calcium carbonate,kaolin, talcum), or it may be a paste and may contain as fourthcomponent inert powdered fillers and optionally as fifth component inertsolvents. Alternatively, it may be a granular product and may containsolid and liquid products as further components. Moreover, it may beused as master batch, and in that case it may additionally contain apolymer (e.g. EPDM) as "binder".

Examples for formulations of the mixtures of the invention containing 40to 50% peroxide are given hereafter:

(a) Liquid formulation

42.8% of 1.1'-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 95%

4.6% 2,5-di-t-amyl hydroquinone

9.3% triallyl mellitate

43.3% dibutyl phthalate

(b) powdered formulations

41.5% dicumyl peroxide, 98%

3.7% 2,5-di-t-amyl hydroquinone, 98%

11.1% triallyl trimellitate

43.7% powdered filler

42.8% alpha,alpha'-bis(t-butylperoxy)diisopropylbenzene, 95%

5.5% 2,5-di-t-amyl hydroquinone, 98%

16.5% triallyl trimellitate

35.2% powdered filler

50.2% 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, 90%

4.2% 2,5-di-t-butyl hydroquinone

12.4% triallyl cyanurate

33.2% powdered filler

(c) pasty formulation

42.8% 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 95%

4.6% 2,5-di-t-amyl hydroquinone

9.3% triallyl trimellitate

23.3% silica

20% paraffin oil, boiling range 300° to 500° C.

(d) granular formulation

42.8% 1,1-bis(t-butylperoxy)-3,3,5,trimethylcyclohexane, 95%

4.6% 2,5-di-t-amyl hydroquinone

9.3% triallyl trimellitate

33% silica

10.3% paraffin oil, boiling range 300° to 500° C.

(e) master batch

50.2% 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, 90%

4.2% 2,5-di-t-butyl hydroquinone

16.5% triallyl trimellitate

19.1% filler

10% EPDM

The formulations of the mixtures according to the invention contain, inaddition to the active ingredients, 0 to 80%, based on the total weightof the mixture, of inert solid or liquid additives. "Inert" in thiscontext is not to be understood in the strict sense of having noinfluence on the active ingredients during storage and crosslinkagesince, for example, acidic fillers decompose a certain portion ofcertain peroxides (such as dicumyl peroxide) in the course ofcrosslinkage and so detract from its crosslinking activity. Examples forliquid inert additives are straight-chain aliphatic, branched aliphatic,alkyl-aromatic oils, oils constituting a mixture of two, three or fourof the above mentioned types, phthalic acid esters or other high-boilingesters. Examples for inert inorganic (usually powdered) additives arecalcium carbonate, kaolin, talcum, calcium silicate, aluminum silicate,fumed silica or precipitated silica. Examples for inert solid or liquidpolymeric additives are ethylene/propylene copolymer (EPR),ethylene/propylene/diene copolymer (EPDM), natural rubber,butadiene/styrene copolymer (SBR), ethylene/vinyl acetate copolymer(EVA), polyethylene (PE), polypropylene (PP), polystyrene, polyvinylchloride (PVC), chlorinated polyethylene, silicone rubber, silicone oil,polyethylene oil and polybutadiene oil. Each one of these additives canbe used per se or in any desired combination with the other additives.

In the process of the invention the polymer is crosslinked with thecombination used according to the invention in the same temperaturerange as with the peroxides alone. The only difference in temperatureresides in the higher starting temperature of the crosslinkage owing tothe combination used according to the invention, which is directlyrelated to the scorch time extension.

The process of the invention may be carried out also with the additionof antiozonants, conventional antioxidants, glass, fibers, pigments,carbon black, UV stabilizers, foaming agents (e.g.2,2'-azobisisobutyramide, 2,2'-azobis(ethyl isobutyrate),azodicarbonamide), flame protection agents, antistatic agents,lubricants, plasticizers or other additives.

The energy requried for crosslinkage can be supplied

(a) by direct contact of the polymer to be crosslinked with a preheatedmaterial, e.g. steel, molten metal, molten salt, water (steam), air,nitrogen;

(b) by electromagnetic radiation, e.g. IR, UHF (=microwaves), X-rays,gamma rays, or

(c) by particle radiation, e.g. alpha rays or beta rays (=electronradiation).

The invention will be explained by the following examples.

EXAMPLES

Agents and inert additives were incorporated into the polymer on a roll.

The following abbreviations are used:

Formulations of the mixtures according to the invention:

A-1

42.8% by wt of 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 95%

4.6% by wt of 2,5-di-t-amyl hydroquinone, 98%

9.3% by wt of triallyl trimellitate

43.3% by wt of silica

B-1

41.5% by wt of dicumyl peroxide, 98%

3.7% by wt of 2,5-di-t-amyl hydroquinone, 98%

11.1% by wt of triallyl trimellitate

43.7% by wt of filler (silica/CaCO₃)

C-1

50.2% by wt of 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, 90%

4.2% by wt of 2,5-di-t-amyl hydroquinone, 98%

12.4% by wt of triallyl trimellitate

33.2% by wt of filler (silica/CaCO₃)

D-1

42.8% by wt of alpha,alpha'-bis(t-butylperoxy)diisopropylbenzene, 95%

5.5% by wt of 2,5-di-t-amyl hydroquinone, 98%

16.5% by wt of triallyl trimellitate

35.2% by wt of filler (silica/CaCO₃).

Standard Peroxide Formulations

A-0: 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 40%, on inertfiller

B-0: dicumyl peroxide, 40%, on inert filler

C-0: 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, 45%, on inert filler

D-0: alpha,alpha'-bis(t-butylperoxy)-diisopropylbenzene, 40%, on inertfiller.

Crosslinkage of the polymers was carried out in the Monsanto-Rheometerin a heated mold with a disk oscillating about 1°. The process ofcrosslinkage was registered in the form of a curve and can be seen fromthe rise of the torque. The curve indicates the scorch time, thecross-linking time, and the crosslinkage (=resultant from degree anddensity of crosslinkage):

M_(L) =lowest point of the crosslinkage curve

M_(H) =highest point of the crosslinkage curve

t₅ =scorch time (=time in minutes until 5% of the maximum crosslinkageof non-inhibited peroxide system is reached)

t_(s) =scorch time of the inhibited systems measured at equal rise ofthe torque (viscosity) of the corresponding non-inhibited peroxide t₅

t₉₀ =crosslinkage time (=time in minutes until 90% of maximumcrosslinkage are reached)

M_(H) -M_(L) =degree of relative crosslinkage.

The Shore hardness is a further indication of rubber crosslinkage.

COMPARATIVE EXAMPLE A

It will be demonstrated that antioxidants (without simultaneous presenceof crosslinkage promoters) deteriorate the cross-linkage (M_(H) -M_(L))and the Shore hardness; the tests proved that the classical antioxidantsBHT and Irganox 1076 cause more deterioration than the hydroquinonederivatives.

Ethylene/Propylene/Diene Copolymer (=EPDM) of the following mixedcomposition

32.5% by wt EPDM of a Mooney viscosity ML (1+4) at 125° C.

1.5% by wt ZnO

29% by wt FEF carbon black

13% by wt CaCO₃

24% by wt paraffinic oil

was crosslinked by means of 3% of 40% dicumyl peroxide at 180° C. in thepresence of 0.3% of the antioxidants 2,6-di-t-butyl-4-methylphenol(=BHT), 3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionic acid stearyl ester(=Irganox 1076), 2-t-butyl hydroquinone, 2,5-di-t-butyl hydroquinone,2-t-amyl hydroquinone, and 2,5-di-t-amyl hydroquinone in the MonsantoRheometer (the percentages 3% and 0.3% relate to the final EPDMmixture):

    ______________________________________                                                         t.sub.s /t.sub.5                                                                     t.sub.90      Shore-A                                 Antioxydant      Min.   Min.   M.sub.H -M.sub.L                                                                     Hardness                                ______________________________________                                        none             0.9    5.1    52     50                                      0.3% BHT         1.3    5.6    35     42                                      0.3% Irganox 1076                                                                              1.1    5.5    36     42                                      0.3% 2-t-butyl-hydroquinone                                                                    1.3    5.1    50     49                                      0.3% 2,5-di-t-butyl-                                                                           1.2    5.2    38     43                                      hydroquinone                                                                  0.3% 2,5-di-t-amyl-                                                                            1.3    5.1    51     49                                      hydroquinone                                                                  0.3% 2-t-amyl-hydroquinone                                                                     1.1    5.1    45     48                                      ______________________________________                                    

EXAMPLE 1

This example demonstrates that it is not possible, by combination of theantioxidants with crosslinkage promoters, to eliminate the negativeeffects (prolonged crosslinkage time (=t₉₀), inferior crosslinkage(=M_(H) -M_(L))) of normal antioxidants (2,6-di-t-butyl-4-methyl phenol(=BHT), 3-(3,5-di-t-butyl-4-hydroxyphenyl)-stearyl propionate (=Irganox1076)), whereas it is possible to eliminate those of the antioxidants ofthe invention (scorch time extender); the data of Comparative Example Apermit further comparison.

The EPDM mixture of Comparative Example A was crosslinked by means of 3%dicumyl peroxide 40% at 180° C. in the presence of 0.3% triallyltrimellitate as crosslinkage promoter and of the antioxidants listed inthe table in the Monsanto Rheometer (the percentages 3 and 0.3% and 0.3%antioxidant below relate to final EPDM mixture):

    ______________________________________                                                         t.sub.s /t.sub.5                                                                     t.sub.90      Shore-A                                 Antioxydant      Min.   Min.   M.sub.H -M.sub.L                                                                     Hardness                                ______________________________________                                        none             0.9    5.1    52     50                                      0.3% BHT         1.3    5.8    42     45                                      0.3% Irganox 1076                                                                              1.1    5.9    42     45                                      0.3% 2-t-butyl-hydroquinone                                                                    1.3    5.1    54     51                                      ______________________________________                                    

EXAMPLE 2

This example shows the scorch-inhibiting effect of 3% of the triplecombinations of the invention as formulations A-1, B-1, C-1, and D-1containing peroxide, scorch time extender, and crosslinkage promoter,compared with the corresponding standard peroxide formulations A-0, B-0,C-0, D-0 containing no scorch time extender and no cross-linkagepromoter, in the crosslinkage of the below indicated EPDM mixture(=ethylene/propylene/diene copolymer):

    ______________________________________                                        32.5%   by wt EPDM with Mooney Viscosity                                                                     (100 parts)                                            ML (1 + 4) at 125° C.                                          1.5%    by wt ZnO              ( 5 parts)                                     29%     by wt FEF carbon black ( 90 parts)                                    13%     by wt CaCO.sub.3       ( 40 parts)                                    24%     paraffinic oil         ( 75 parts)                                                                   310 parts                                      ______________________________________                                        Crosslinkage                                                                           t.sub.s /t.sub.5                                                                     t.sub.90       Crosslinkage                                                                           Shore-A                               Formulation                                                                            Min.   Min     M.sub.H -M.sub.L                                                                     Temperature                                                                            Hardness                              ______________________________________                                        A-O   3.0%   0.34   1.13  5      180° C.                                                                         63                                  A-1   3.0%   0.68   1.17  5.9    180° C.                                                                         63                                  B-0   3.0%   0.65   4.0   8.75   180` C.  67                                  B-1   3.0%   1.15   4.3   9.3    180° C.                                                                         67                                  C-0   2.5%   0.70   6.5   11.4   180° C.                                                                         73                                  C-1   2.5%   1.55   5.5   11.4   180° C.                                                                         73                                  D-0   2.0%   0.70   6.5   9.05   180° C.                                                                         70                                  D-1   2.0%   1.60   6.6   9.25   180° C.                                                                         70                                  ______________________________________                                    

The scorch times (=t_(s)) of the formulations (A-1, B-1, C-1, D-1)according to the invention are about twice as long as those t₅ providedby the standard peroxide formulations (A-0, B-0, C-0, D-0) containingthe same amount of peroxide at virtually equal crosslinkage time (=t₉₀)and with equal or better crosslinkage (=M_(H) -M_(L)).

EXAMPLE 3

This example shows the scorch-inhibiting effect of 2% and 3% of thetriple combination according to the invention as formulation B-1, C-1,and D-1 containing peroxide, scorch time extender, and crosslinkagepromoter, in comparison with the corresponding standard peroxideformulations B-0, C-0, and D-0 containing no scorch time extender and nocrosslinkage promoter, in the crosslinkage of HDPE (=high densitypolyethylene) in the Monsanto Rheometer at 150° C. and 180° C.:

    ______________________________________                                        Crosslinkage    t.sub.s /t.sub.5                                                                     t.sub.90      Crosslinkage                             Formulation     Min.   Min    M.sub.H -M.sub.L                                                                     Temperature                              ______________________________________                                        B-0     3%      1.0    4.0    94     180° C.                           B-1     3%      1.4    4.0    112    180° C.                           B-0     3%      4.0    56.0   --     150° C.                           B-1     3%      6.0    55.0   --     150° C.                           C-0     2%      1.35   6.0    106.5  180° C.                           C-1     2%      1.9    6.0    110.0  180° C.                           C-0     2%      4.5    45.0   40.5   150° C.                           C-1     2%      9.0    48.0   42.5   150° C.                           D-0     2%      1.3    6.0    106.0  180° C.                           D-1     2%      1.8    6.3    103.0  180° C.                           D-0     2%      5.5    70.0   50.0   150° C.                           D-1     2%      10.5   76.0   46.0   150° C.                           ______________________________________                                    

The scorch times (=t_(s)) of the formulations of the invention (B-1,C-1, D-1) are about 1.5 to 2 times longer than the t₅ of the standardperoxide formulations (A-0, B-0, C-0, D-0) in substantially equalcrosslinking time (=t₉₀) and at substantially equal degree ofcrosslinkage (=M_(H) -M_(L)).

EXAMPLE 4

This example demonstrates the scorch-inhibiting effect of 2% and 3.5%and 4% and 4.5% of the triple combination according to the invention asformulation A-1, B-1, C-1, D-1 containing peroxide, scorch timeextender, and crosslinkage promoter, compared with the correspondingstandard peroxide formulations A-0, B-0, C-0, and D-0 containing noscorch time extender and no crosslinkage promoter, in the crosslinkageof EVA (=ethylene/vinyl acetate copolymer) in the Monsanto-Rheometer at150° C. and 180° C.:

    ______________________________________                                        Crosslinkage    t.sub.s /t.sub.5                                                                     t.sub.90      Crosslinkage                             Formulation     Min.   Min    M.sub.H -M.sub.L                                                                     Temperature                              ______________________________________                                        A-0     4.5%    1.5    8.6    28.0   150° C.                           A-1     4.5%    3.0    7.2    39.5   150° C.                           A-0     4.5%    0.65   1.7    20.5   180° C.                           A-1     4.5%    1.1    1.72   21.0   180° C.                           B-0     2%      8.0    70.0   21.0   150° C.                           B-1     2%      15.0   73.0   30.0   150° C.                           B-0     3.5%    1.25   5.8    55.0   180° C.                           B-1     3.5%    2.0    5.5    55.0   180° C.                           C-0     4%      8.0    129.0  84.0   150° C.                           C-1     4%      18.5   132.0  93.0   150° C.                           C-0     4%      1.6    9.6    86.0   180° C.                           C-1     4%      2.5    9.3    93.0   180° C.                           D-0     2%      12.0   132.0  41.0   150° C.                           D-1     2%      33.0   132.0  41.0   150° C.                           D-0     3.5%    1.7    9.2    80.0   180° C.                           D-1     3.5%    2.8    8.6    79.0   180° C.                           ______________________________________                                    

The scorch times (=t_(s)) of the formulations of the invention (A-1,B-1, C-1, D-1) are about 1.5 to 2 times longer than the t₅ of thestandard peroxide formulations containing the same amount of peroxide(A-0, B-0, C-0, D-0), while the crosslinking time (=t₉₀) issubstantially equal and the degree of crosslinkage (=M_(H) -M_(L)) issubstantially equal.

EXAMPLE 5

This example shows the good storage stability of the triple combinationsA-1, B-1, C-1 and D-1 according to the invention containing peroxide,scorch time extender, and crosslinkage promoter, in comparison with thecorresponding standard peroxide formulations A-0, B-0, C-0, and D-0containing peroxide, but no scorch time extender and no crosslinkagepromoter, and the formulations A-2, B-2, C-2 and D-2 containing peroxideand crosslinkage promoter, but no inhibitor, and the formulations A-3,B-3, C-3, and D-3 containing peroxide and antioxidant, but nocross-linkage promoter. The said formulations are stored in a water bathof 60° C. in a closed vessel for 10 days. The peroxide content isdetermined before and afterwards by means of the customary standardanalysis methods. This primarily demonstrates that the stability of theperoxide is not impaired by antioxidant and crosslinkage promoter.

    ______________________________________                                        Storage Stability (10 days at 60° C.):                                             Content                                                                              Decrease in Peroxide                                       Peroxide Initial  after    (a) Difference                                                                          (b) in % of                              Formulation                                                                            Content  10 Days  Initial Content                                    ______________________________________                                        A-0      38.7%    30.8%     9.6%     23.8%                                    A-1      44.3%    38.8%     5.5%     12.4%                                    A-2      42.1%    25.6%    16.5%     39.2%                                    A-3      41.8%    36.0%     5.8%     13.9%                                    B-0      39.8%    39.8%     --       no decrease                              B-1      38.4%    39.1%     --       no decrease                              B-2      40.1%    35.8%     4.3%     10.7%                                    B-3      40.1%    38.0%     1.9%      4.7%                                    C-0      46.0%    45.0%     1%        2.17%                                   C-1      45.2%    45.3%     --       no decrease                              C-2      45.2%    40.0%     5.2%     11.50%                                   C-3      46.0%    44.0%     2.0%      4.3%                                    D-0      40.7%    41.0%     --       no decrease                              D-1      39.1%    39.2%     --       no decrease                              D-2      40.2%    35.0%     5.2%     12.4%                                    D-3      40.3%    39.5%     0.8%      2.0%                                    ______________________________________                                    

The excellent stability of the mixed crosslinking agents was surprisingsince normally mixtures consisting of polyfunctional monomers andperoxides at this concentration are very unstable. Also peroxides mixedwith hydroquinone derivatives are less stable, as is evident from thesestability tests. The listed products (A-D)-2 and (A-D)-3 have thefollowing composition:

    ______________________________________                                        A-2 = 43%     by wt of 1,1-bis(t-butylperoxy)-3,3,5-trimethyl                               cyclohexane, 95%                                                      10%     by wt of triallyl trimellitate                                        47%     by wt of silica                                                 A-3 = 43%     by wt of 1,1-bis(t-butylperoxy)-3,3,5-trimethyl                               cyclohexane, 95%                                                      5%      by wt of 2,5-di-t-amyl hydroquinone, 98%                              52%     by wt of silica                                                 B-2 = 41.5%   by wt of dicumyl peroxide, 98%                                        11.1%   by wt of triallyl trimellitate                                        43.4%   by wt of filler (silica/CaCO.sub.3)                             B-3 = 41.5%   by wt of dicumyl peroxide, 98%                                        4%      by wt of 2,5-di-t-amyl hydroquinone, 98%                              54.5%   by wt of filler (silica/CaCO.sub.3)                             C-2 = 50.2%   by wt of 2,5-dimethyl-2,5-bis(t-butylpeproxy)-                                hexane, 90%                                                           12.4%   by wt of triallyl trimellitate, 98%                                   37.4%   by wt of silica (filler)                                        C-3 = 50.2%   by wt of 2,5-dimethyl-2,5-bis(t-butylperoxy)-                                 hexane, 90%                                                           4.2%    by wt of 2,5-di-t-amyl hydroquinone, 98%                              55.6%   by wt of filler (silica)                                        D-2 = 42.8%   by wt of α,α'-bis(t-butylperoxy)-diisopropyl-                     benzene, 95%                                                          16.5%   by wt of triallyl trimellitate, 98%                                   30.7%   by wt of filler (silica/CaCO.sub.3)                             D-3 = 42.8%   by wt of α,α'-bis(t-butylperoxy)-diisopropyl-                     benzene, 95%                                                          5.5%    by wt of 2,5-di-t-amyl hydroquinone, 98%                              51.7%   by wt of filler (silica/CaCO.sub.3)                             ______________________________________                                    

EXAMPLE 6

Example 6 shows that in the triple combinations A-1, B-1, C-1 and D-1according to the invention not only the peroxide but also thecrosslinkage promoter and the antioxidant are unaffected by storage,since the activity according to the invention of the triple combinationwith respect to scorch time, crosslinkage and crosslinking rate wasequal to that at the start of the storage period.

The mixtures A-1, B-1, C-1 and D-1 of the invention were stored for 6months at room temperature. They were tested for their activity incrosslinking EPDM, HDPE, and EVA after 2 months and after 6 months, asdescribed in Examples 2, 3 and 4. The measured values were the same asthose of the freshly prepared mixtures.

EXAMPLE 7

This example shows that the results obtained in the cross-linkageexamples 2, 3 and 4 are obtained also when the peroxide, antioxidant,corsslinkage promoter, and filler components are added individually andin succession, rather than in the form of the batches A-1, B-1, C-1, andD-1 of the invention, to the polymers to be cross-linked prior tocrosslinkage. According to this "piece-meal procedure" only a part ofExamples 2, 3 and 4 was repeated, namely

Example 2:

A-1 (mixture) compared with A-1 (individual)

Example 3:

B-1 (mixture) compared with B-1 (individual)

Example 4:

C-1 (mixture) compared with C-1 (individual)

D-1 (mixture) compared with D-1 (individual)

The results obtained with (A-D)-1 (mixture) were identical to thoseobtained with (A-D)-1 (individual).

We claim:
 1. A mixture suitable for crosslinking polymers providingreduced scorch during compounding consisting essentially of:(a) at leastone organic peroxide suitable for crosslinking polymers, said peroxidebeing at least 40% by weight of said mixture, and being selected fromthe group consisting of dialkyl peroxides, ketal peroxides, peresters,dialkyl peroxide perketals, diaryl peroxides, monoperoxy carbonates andmixtures thereof; (b) at least one hydroquinone derivative selected fromthe group represented by the general formulas I, II and III ##STR4##wherein R and R¹, which may be the same or different, represent H,t-alkyl, t-cycloalkyl, or aryl-t-alkyl and X represents C₁₋₁₈ -alkylenewhich may be substituted by substituents selected from among the groupsconsisting C₁₋₁₀ -alkyl, C₁₋₁₀ -alkoxy, phenyl and mixtures thereof,C₂₋₁₈ alkenylene, or ₂₋₁₈ -alkpolyenylene both of which may besubstituted with substituents selected from among the group consistingof C₁₋₁₀ -alkyl, --(CR² R³)_(n) --AR--(CR² R³)_(m) -- and mixturesthereof wherein R₂ and R₃ may be the same or different and representhydrogen or C₁₋₁₀ -alkyl, Ar represents phenyl or naphthyl, n and m mayeach vary from 0 to 18, and n+m≦18, and the hydroquinones of the formulaIII may be substituted by C₁₋₁₀ -alkyl on the benzene ring of thenaphthalene structure not substituted by the hydroxy groups and R; and(c) at least one crosslinkage promoter.
 2. A mixture according to claim1, wherein the organic peroxide is dialkyl peroxide, diaralkyl peroxide,or ketal peroxide.
 3. A mixture according to claim 1, wherein thehydroquinone derivative is mono-t-butyl hydroquinone, 2,5-di-t-butylhydroquinone, or 2,5-di-t-amyl hydroquinone.
 4. A mixture according toclaim 2, wherein the hydroquinone derivative is mono-t-butylhydroquinone, 2,5-di-t-butyl hydroquinone, or 2,5-di-t-amylhydroquinone.
 5. A mixture according to claim 1, wherein thecrosslinkage promoter is triallyl cyanurate, triallyl isocyanurate, ortriallyl trimellitate.
 6. A mixture according to claim 2, wherein thecrosslinkage promoter is triallyl cyanurate, triallyl isocyanurate, ortriallyl trimellitate.
 7. A mixture according to claim 3, wherein thecrosslinkage promoter is triallyl cyanurate, triallyl isocyanurate, ortriallyl trimellitate.
 8. A mixture according to claim 4, wherein thecrosslinkage promoter is triallyl cyanurate, triallyl isocyanurate, ortriallyl trimellitate.
 9. A mixture suitable for crosslinking polymersas defined in claim 1 additionally consisting essentially of fillersselected from the group consisting of inert liquids, inert solids,polymeric binders or mixtures thereof.